Metal Halide Surface Treatment of Quantum Dots

Quantum dot (QD) solids are a solution-processed, composite thin film semiconductor system that is being developed for optoelectronics (display technology, solid state lighting, next generation photovoltaics, photodetector application, etc.). For photovoltaics, in addition to being solution processable, QD solar cells (QDSCs) have a higher limiting single junction power conversion efficiency than is possible using conventional bulk or thin film semiconductors due to enhanced multiple exciton generation in the QDs. Recent improvements in QDSC performance and processing ease have resulted from modification of the device architecture, processing of the QD-layers under ambient conditions, improved QD synthetic procedures and surface treatments improving QD passivation.

Description

Scientists at the National Renewable Energy Laboratory (NREL) have created a method of preparing fully inorganic arrays of PbE (E=S or Se) QD films using a metal halide (PbI2, PbCl2, CdI2, or CdCl2) – dimethylformamide (DMF) complex to displace oleate ligands and terminate the QDs with the metal halide. The resulting QD-solids have a significant reduction in the carbon content compared to films treated with thiols and organic halides. NREL researchers have found that this treatment is successful in removing alkyl surface ligands and also replaces any surface bound impurities. This treatment protocol results in stoichiometric QD films with a deeper work function and band positions than other ligand exchanges. The method developed produces solar cells that perform well even at film thicknesses approaching a micron, indicating improved carrier transportation in the QD films. The best QD solar cells based on PbI2 had power conversion efficiencies above 8%.